Hell Divers by Nicholas Sansbury Smith

But turning back to the Vanguard Islands without a new food source could doom all humanity to starvation. As losses from both missions mount, the king wonders if finding this place is worth the cost. On Australia's Sunshine Coast, King Xavier leads the search for the Coral Castle while Captain Rolo deploys the Hell Divers to hunt for a new source of seeds. The only hope of defeating it is a youth who has learned to survive in the deadly ruins. At Panama, a new leader emerges as an ancient evil threatens Outpost Gateway. And with King Xavier off in search of the Coral Castle, he feels the target on his back.

When two sky people are murdered, Chief Michael Everhart knows the stage is set for mutiny. The rest of the fleet returns to the Vanguard Islands with new ships and supplies, but tensions rise with rumors of a hidden food cache. The battle for Panama is over, and King Xavier Rodriguez secretly deploys the airship Vanguard to Queensland.